1. Field of Invention
The invention relates generally to a system for purifying a liquid, and specifically to (1) membrane treatment processes used in a water purification system, and (2) a microprocessor controlled operating system to improve the performance of a water purification system utilizing reverse osmosis (RO) filtration and ultra-violet light (UV) treatment.
2. Description of Prior Art
Domestic point-of-use (POU) water treatment technologies have been developed to improve the quality of drinking water supplies, and are used to process a small fraction (one to two percent) of supplied tap water. Various POU treatment technologies improve drinking water quality. Reverse osmosis filtration is a widely used and versatile separation process, and it is regarded as one of the most effective processes for providing quality drinking water.
The use of very fine RO membranes is, however, subject to a number of constraints. In general, RO membranes should be used in conjunction with other filtration elements. In fact, RO membrane units should be configured in a treatment system that must also provide for the pre-filtration of tap water, the production and storage of RO product water, and the post-filtration of the stored RO water prior to deliver via a faucet. RO water storage is needed because household water is supplied only at low pressure, that is, 45-60 pounds per square inch (psi). At low pressures, RO product water cannot be produced at high delivery flow rates that can directly supply a faucet. The large surface area membranes that would be required are costly and not practicable for POU applications.
In prior art RO systems, the energy required to run an RO system is supplied indirectly by the hydraulic pressure in the water supply line. While this mode of RO system operation (which does not require a supply of electricity) is convenient, it nevertheless imposes a number of constraints on the performance of prior art systems. In particular, the use of the water supply line pressure to drive the water purification system limits the operational performance of the RO membrane itself. Three variable factors that can adversely affect an RO membrane operation are (1) the incoming water supply pressure, (2) the membrane flux ratio (defined by the ratio of the membrane reject flow to the product water flow through the membrane), and (3) the back pressure transmitted to the membrane from the storage device. Virtually all prior art RO systems use a bladder tank for the storage of RO product water. The working pressure of the RO membrane is the supply pressure to the membrane unit less the bladder tank pressure that must be overcome to force more water into the tank. High internal tank pressure, when the tank is full, permits a good water delivery rate to a faucet but a poor RO production rate when the RO water is stored. When the tank storage is low, water delivery from the bladder tank is poor, but RO water production to the bladder tank is high. This arrangement for operating prior art RO systems results in a constant compromise in system performance.
The performance limitations of elementary RO systems are basically caused by (1) intermittent or low water supply pressure, (2) inactivity of the system during periods when water is not drawn from the system, storage tank back pressure, and (3) the operational use of less than recommended flux ratios. Low flux ratios are used to reduce high water wastage, but the arbitrary reduction of reject flows can lead to deterioration of treatment elements in the system. Performance limitations have been addressed by providing optional add-on components to enhance system performance. These extra components include pressure booster pumps, shut-off valves and devices to utilize the energy lost in the reject flow. The basic design and operation of prior art systems has not been changed by the addition of these technical fixes.
An advanced prior art system for domestic water purification is illustrated in FIG. 1. A branching feed valve 10 connects the system to a pressurized supply of feed water. A conduit 12 connects the feed valve 10 to a pre-filter 14, which removes contaminants such as suspended particles and dissolved chemical contaminants which can have a detrimental effect on the subsequent membrane treatment. Pre-filters are typically designed to capture particles greater than five microns in size. After the pre-filter 14, the water is passed through a carbon filter 16 which adsorbs organic or chlorinated chemicals in the water. From the carbon filter 16 the water is passed via a shut-off valve assembly 18 to the membrane unit 20. With cross-flow filtration, the Reverse Osmosis membrane contained in the membrane unit 20 will remove macromolecules and ionic particles from the pre-filtered water, and the concentrated contaminated stream can be directed through outlet 21 to a drain through a flow restriction valve 22 and a waste conduit 24. The purified product water or permeate is directed through outlet 26, and along conduit 28 which is provided with a check valve 30. The product water then passes through the shut-off valve assembly 18 and flows on to the storage tank 32. A typical storage tank 32 is a bladder tank. An air chamber in the tank is compressed as the tank fills with water from the membrane unit 20, and the resulting air compression causes pressurization in the water compartment of the bladder tank. The bladder tank pressure induced by the storage of treated water produces flow at the faucet 34 when the faucet is opened. The stored water is forced into conduit 36 and through the carbon filter 38 which provides a final "polishing" of the dispensed water. A pressure pump 40 and a booster permeate pump 42, which can use the waste water line pressure to boost the pumping pressure, can be configured as optional extras to improve the performance. Water conductivity sensors 44 in conduit 12, and 46 in conduit 36, are used in advanced prior art systems to monitor the relative removal of mineral ions by the Reverse Osmosis membrane 20. This monitoring through the paired sensors 44 and 46 provides a relative indication of water quality improvement, which is used to display the proper functioning of the RO water purification system.